Cause of Extreme Heavy and Persistent Rainfall over Yangtze River in Summer 2020

Abstract

Record-breaking heavy and persistent precipitation occurred over the Yangtze River Valley (YRV) in June–July (JJ) 2020. An observational data analysis has indicated that the strong and persistent rainfall arose from the confluence of southerly wind anomalies to the south associated with an extremely strong anomalous anticyclone over the western North Pacific (WNPAC) and northeasterly anomalies to the north associated with a high-pressure anomaly over Northeast Asia. A further observational and modeling study has shown that the extremely strong WNPAC was caused by both La Niña-like SST anomaly (SSTA) forcing in the equatorial Pacific and warm SSTA forcing in the tropical Indian Ocean (IO). Different from conventional central Pacific (CP) El Niños that decay slowly, a CP El Niño in early 2020 decayed quickly and became a La Niña by early summer. This quick transition had a critical impact on the WNPAC. Meanwhile, an unusually large area of SST warming occurred in the tropical IO because a moderate interannual SSTA over the IO associated with the CP El Niño was superposed by an interdecadal/long-term trend component. Numerical sensitivity experiments have demonstrated that both the heating anomaly in the IO and the heating anomaly in the tropical Pacific contributed to the formation and maintenance of the WNPAC. The persistent high-pressure anomaly in Northeast Asia was part of a stationary Rossby wave train in the midlatitudes, driven by combined heating anomalies over India, the tropical eastern Pacific, and the tropical Atlantic.

Markov Chain Analysis of Rainfall over East Asia: Unusual Frequency, Persistence, and Entropy in the Summer 2020

“…8e) is also of note, suggesting a role of the Indian Ocean and western Pacific. This analysis is consistent with some recent studies, which conclude that the extremely strong Northwest Pacific anticyclone anomaly accompanied by both La-Niña like SST forcing in tropics and intensified Indian Ocean warming is responsible for extreme rainfall in summer 2020 (Fang et al 2021;Pan et al 2021;Tang et al 2021). However, further analysis is required to quantify the relative importance of the observed SST anomalies.…”

East Asian summer monsoon enhanced by COVID-19

“…The observed seasonal anomaly in a summer is strongly affected by the interannual climate variability. Indeed, the record-breaking extreme rainfall along the Yangtze River valley in 2020 was primarily attributed to the interannual variability of the climate system associated with tropical SST anomalies (Pan et al 2021;) and atmospheric internal dynamics (Liu et al 2021), particularly the warm SST anomaly over the Indian Ocean associated with a decaying El Niño event ) and the cold SST anomaly over equatorial Paci c associated with a developing La Niña event (Pan et al 2021). Until now, we have experienced two summers after the COVID-19 outbreak, i.e., 2020 (following an El Niño event in the preceding winter) and 2021 (following a La Niña event in the preceding winter).…”

A curious case of the Indian Summer Monsoon 2020: The influence of Barotropic Rossby Waves and the monsoon depressions

“…In this study, we investigate the reasons behind the failure of LPAs to intensify into MDs during August 2020 and factors that are responsible for anomalously high rainfall over Indian subcontinent. A significant large-scale feature during the ISM 2020 is the development of La Niña in the Pacific Ocean (e.g., Chu et al, 2021;Pan et., 2021 andQiao et al, 2021), cooccurring with basin-wide warming in the Indian Ocean. Numerous studies on the El Niño Southern Oscillation (ENSO) -monsoon relationship indicate that the ISM rainfall is significantly modulated by the teleconnections between the air-sea interaction processes of global oceans (e.g., Sikka 1980;Pant and Parthasarathy 1981).…”